Abstracts

Objective: The purpose of this study is to determine the accuracy of MEG in localizing epileptogenic foci in patients with epilepsy.
MEG may have several advantages over EEG in determining epileptogenic focus. It is less influenced by differences in conductivities of various types of brain tissues.
METHODS: 480 patients underwent MEG testing at Memorial Hermann Hospital between July 1997 until April 2002. In over 95% of patients, MEG study was performed as a part of pre-surgical evaluation for epilepsy. During routine preoperative interictal MEG evaluation in these patients, ictal MEG was fortuitously recorded in 11 patients. In all patients interictal MEG evaluation was performed using 148 channel whole head magnetometer with simultaneous EEG recording. Ictal MEG studies were compared to scalp ictal EEG recordings.
RESULTS: Of the 11 ictal MEG studies reviewed, only five showed reliable recordings adequate for interpretation. All patients also had scalp ictal EEG recordings. Three out of five patients with reliable ictal MEG recordings also underwent intracranial electrode evaluation for further delineation of the epileptogenic focus. One of these patients had mesial temporal lobe epilepsy (MTLE) confirmed by intracranial recording. Ictal scalp EEG lateralized to the same hemisphere, but was not well localized at onset. Ictal MEG dipoles in this patient showed a focal temporal onset. The other four had extratemporal onset of seizures. In three out of four patients with extratemporal epilepsy, ictal MEG localization was similar to scalp ictal EEG recordings. In the fourth patient, ictal MEG spikes were localized to the left mesial frontal region. However scalp and intracranial electrode evaluation revealed an indistinct, diffuse EEG onset.
CONCLUSIONS: These results suggest that ictal MEG studies may:
1. Confirm scalp ictal EEG recordings
2. Further define the epileptogenic zone
3. Reveal a focal onset not seen with scalp EEG.